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$PGGGA and NMEA standard

 
 
Henryk Birecki
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      02-03-2005, 04:21 PM
Can anyone tell me how exactly the $GPGGA sentence is defined in NMEA
standard. Specifically which altitude is it supposed to report? MSL or
height over ellipsoid? Currently there are devices on the market that
do both. Garmin and some others reports MSL altitude, SiRF chipset
devices - altitude over ellipsoid.

Thanks.

Henryk Birecki

 
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RumJungle
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      02-03-2005, 05:29 PM
I have observed this as well. Looking at my NMEA 0813 Specification,
the GGA sentence shows that the elevation should be MSL. Take a look at
Dale's pages on NMEA sentences:


http://www.gpsinformation.org/dale/nmea.htm#GGA


Monte---

 
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Eino Uikkanen
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      02-03-2005, 06:02 PM
"Henryk Birecki" <(E-Mail Removed)> kirjoitti viestissä
news:(E-Mail Removed)...
> Can anyone tell me how exactly the $GPGGA sentence is defined in NMEA
> standard. Specifically which altitude is it supposed to report? MSL or
> height over ellipsoid? Currently there are devices on the market that
> do both. Garmin and some others reports MSL altitude, SiRF chipset
> devices - altitude over ellipsoid.
>
> Thanks.
>
> Henryk Birecki


According to the specifications I have been using, you should get both
height above MSL and WGS84 geoid height.
The height above (WGS84) ellipsoid is the sum of those two.

Height above ellipsoid = Altitude above MSL (field 9) + WGS84 geoid height
(field 11)

' $GPGGA - Global Positioning System Fix Data
' $GPGGA,155537,6006.718,N,02426.290,E,1,05,2.4,50.5 ,M,19.7,M,,*79
' 1 123519 Fix taken at 12:35:19 UTC
' 2,3 4807.038,N Latitude 48 deg 07.038' N
' 4,5 01131.324,E Longitude 11 deg 31.324' E
' 6 1 Fix quality: 0 = invalid
' 1 = GPS fix
' 2 = DGPS fix
' 7 08 Number of satellites being tracked
' 8 0.9 Horizontal dilution of position
' 9,11 545.4,M Altitude, Metres, above mean sea level
' 11,13 46.9,M Height of geoid (mean sea level) above WGS84 ellipsoid
' 13 (empty field) time in seconds since last DGPS update
' 14 (empty field) DGPS station ID number

Eino Uikkanen
http://www.kolumbus.fi/eino.uikkanen/gb/index.htm


 
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RumJungle
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      02-03-2005, 06:28 PM
Based on the above, this indicates that SiRF is not truly NMEA
compliant for the GGA message. They have seemed to omit the Height of
geoid (field 11) and placed the Height above ellipsoid in field 9 where
this field should be MSL.

As Dale stated in his web site, the altitude should be suspect if the
height of geoid is missing.


Monte---

 
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Sam Wormley
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      02-03-2005, 08:48 PM
Henryk Birecki wrote:
> Can anyone tell me how exactly the $GPGGA sentence is defined in NMEA
> standard. Specifically which altitude is it supposed to report? MSL or
> height over ellipsoid? Currently there are devices on the market that
> do both. Garmin and some others reports MSL altitude, SiRF chipset
> devices - altitude over ellipsoid.
>
> Thanks.
>
> Henryk Birecki
>




1 2 3 4 5 6 7 8 9 10 11 12 13 14
------|---------|--------|-|---------|-|-|--|---|-----|-|----|-|---|---
"$GPGGA,114114.22,4202.007,N,09339.927,W,2,04,1.5, 312.8,M,31.4,M,0.0,04d"

Where 1 UTC: 11h41m14.22s
2,3 LAT: 42d02.007m N
4,5 LONG: 093d39.927m W
6 GPS: 2 (GPS Quality, 0=invalid, 1=GPS fix, 2=Diff GPS fix)
7 SATS: 04
8 HDOP: 1.5
9,10 ALT: 312.8 meters (1026.25 ft) Orthometric HEight (MSL)
11,12 GEOID 31.4 meters (Geoidal Separation)
13 AGE: 0.0 Seconds since last update from diff. ref. station
14 ID: 04d



-Sam Wormley
http://edu-observatory.org/gps/height.html
 
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Sam Wormley
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      02-03-2005, 09:01 PM
Sam Wormley wrote:
> Henryk Birecki wrote:
>
>> Can anyone tell me how exactly the $GPGGA sentence is defined in NMEA
>> standard. Specifically which altitude is it supposed to report? MSL or
>> height over ellipsoid? Currently there are devices on the market that
>> do both. Garmin and some others reports MSL altitude, SiRF chipset
>> devices - altitude over ellipsoid.
>>
>> Thanks.
>>
>> Henryk Birecki
>>

>
>
>
> 1 2 3 4 5 6 7 8 9 10 11 12 13 14
> ------|---------|--------|-|---------|-|-|--|---|-----|-|----|-|---|---
> "$GPGGA,114114.22,4202.007,N,09339.927,W,2,04,1.5, 312.8,M,31.4,M,0.0,04d"
>
> Where 1 UTC: 11h41m14.22s
> 2,3 LAT: 42d02.007m N
> 4,5 LONG: 093d39.927m W
> 6 GPS: 2 (GPS Quality, 0=invalid, 1=GPS fix, 2=Diff GPS
> fix)
> 7 SATS: 04
> 8 HDOP: 1.5
> 9,10 ALT: 312.8 meters (1026.25 ft) Orthometric HEight (MSL)
> 11,12 GEOID 31.4 meters (Geoidal Separation)
> 13 AGE: 0.0 Seconds since last update from diff. ref.
> station
> 14 ID: 04d
>
>
>
> -Sam Wormley
> http://edu-observatory.org/gps/height.html

 
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Dale DePriest
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      02-03-2005, 10:29 PM
If the message includes the Geoid difference data then the altitude is
referenced to MSL. If the Geoid difference data is missing then the unit
doesn't have this data and can only report Ellipsoid altitude data.
Check my web site for a discussion of this.

Dale

Henryk Birecki wrote:

> Can anyone tell me how exactly the $GPGGA sentence is defined in NMEA
> standard. Specifically which altitude is it supposed to report? MSL or
> height over ellipsoid? Currently there are devices on the market that
> do both. Garmin and some others reports MSL altitude, SiRF chipset
> devices - altitude over ellipsoid.
>
> Thanks.
>
> Henryk Birecki
>


--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs
 
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Hannes Fikis
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      02-11-2005, 10:18 PM
Henryk Birecki <(E-Mail Removed)> wrote in message news:<(E-Mail Removed)>. ..
> Can anyone tell me how exactly the $GPGGA sentence is defined in NMEA
> standard. Specifically which altitude is it supposed to report? MSL or
> height over ellipsoid? Currently there are devices on the market that
> do both. Garmin and some others reports MSL altitude, SiRF chipset
> devices - altitude over ellipsoid.
>
> Thanks.
>
> Henryk Birecki


Unfortunately the SIRF chipset (at least V2.32) is not fully compliant
to $GPGGA.

Example:
$GPGGA,143429.726,1122.6419,N,02211.7149,E,0,00,50 .0,300.5,M,43.5,M,0.0,0000*40

If you would get such a message, it would mean that you are 300.5m
above mean-sea-level and the "geodial separation" (= the difference
between the WGS-84 earth ellipsoid and mean-sea-level) is 43.5m at
your position. That is what NMEA specification says.

But if you use a SRIF-unit, the same message would mean that you are
262m (=305.5-43.5) above mean-sea-level.

Not so long ago, SRIF-units couldn´t provide the "geoidal seperation".
Now they do, but they do that in a way, which is absolutely not NMEA
compliant.

Hannes Fikis

PS: see also: http://www.gpspassion.com/forumsen/t...TOPIC_ID=10915
 
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